The present invention relates to a method and apparatus for measuring the viscosity of fluids, and more particularly, to a noncontact method of measuring frequency and decay of induced oscillation in a fluid surface. The sprayability of paint is largely determined by surface tension (.sigma.) and viscosity (.eta.). On-line monitoring of .sigma. and .eta. is important because these properties fluctuate over time. If these fluctuations are ignored, a less than optimal application of the paint is achieved.
Presently, liquid paint viscosity in most industrial plants is determined manually at the beginning of each working day by collecting samples of paint and measuring the time it takes for the samples to flow through a calibrated orifice or a constriction in a tube. However, this method does not easily lend itself to automated, on-line, noncontact monitoring of .sigma. and .eta.. Other conventional methods employed to measure the viscosity of fluids also require contact with the fluid. Such methods include the falling-sphere method, capillary tube method, rotating cylinder method, and disk method, as well as ultrasonic wave velocity and attenuation methods.
It is possible to determine the viscosity of a fluid in a noncontact manner by measuring the Brillouin scattering of laser light. The line widths of Brillouin-scattered light are directly related to the viscosity of the fluid. The disadvantage of using Brillouin scattering for measuring the viscosity of paint is the opacity of paint to light, and the corresponding high intensities of laser light required to observe Brillouin scattering.
Accordingly, there exists a need for a rapid, on-line, noncontact method that measures the viscosity of a liquid, such as paint. It is also desirable to measure the surface tension of the fluid, because for certain paints, for example, there is also a high correlation between paint sprayability and surface tension. It is therefore desirable to have a non-contact method of measuring paint viscosity to obviate clogging of test components to reduce maintenance costs.